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Re: The QHA antenna.



[This is a somewhat technical message about satellite antennas, and those on
AMSAT-BB not interested in homebrewing or modeling may wish to skip to the
next message.]

   I've looked at that design, even that particular page, and several others and
   have not thought the QHA would be that easy to build.  This one does appear
   simpler than the more common coax-element ones.  I'm really glad to hear
   several people say they have done it and it wasn't overly difficult.  Even
   Martin, KB2UC, favorably reviews it in his books.  I have been working over
   the last several months on a couple of different designs trying to find that
   elusive "perfect" omni antenna.  Maybe it's been there all along and I was
   just too thick-headed to pay attention.  I usually build a 70 cm prototype
   first since I can easily use the FO-20/29 downlinks as my test range.  I
   recently had a dismal failure of a design I spend several weeks modeling and
   perfecting, only to determine it was far inferior to an eggbeater (my standard
   reference antenna) in actual use.  Even field-strength measurements defy my
   computer models!  I'm begining to suspect the computers are plotting against
   me :-)

Yeah, Jerry, i'm having trouble modeling this, also.  And the computers
should be plotting for you, not against you...  <chuckle>

I want to experiment with different lengths and frequencies, but i'm also
having difficulty with trying to model this.   The confusion i'm running 
into is how to determine the input impedance of the model given at:

   http://www.personal.u-net.com/~pilotltd/nec.htm

On lines 24 and 26, i see one loop being excited with a voltage in the real
domain and the other with one in the imaginary domain, as explained in the
manual:

   Voltage source (Il = 0 or 5) (Fl) - Real part of the voltage in
   volts. (F2) - Imaginary part of the voltage in volts. (F3) - If a one
   is placed in column 20 (see above), this field can be used to specify
   a normalization constants for the impedance printed in the optional
   impedance table. Blank in this field produces normalization to the
   maximum value. (F4), (F5), & (F6) - Blank. 

			 [NEC2 User's Guide (see attribution below), p. 49]

I don't understand having one real excitation in one loop and an 'imaginary'
excitation in another is supposed to do.  The results don't make that much 
sense to me, either.  (See below.)

By the way, those columns weren't labeled, the fourth column isn't labeled
correctly as i understand it.

   I3 = m, specifies the m th segment of the set of segments whose tag
   numbers are equal to the number set by the previous parameter. If the
   previous parameter is zero, the number in (I3) must be the absolute
   segment number of the source.
							   [ibid. p. 48]
Here's what i'm getting:

       http://www.qsl.net/kd6pag/nec/apt1a.html

which includes the NEC2 'deck' at the bottom of the page. [n.b.  CW lovers
might take note that the first serious computer i used was a batch system
utilizing 'IBM cards' and a "Model 33" teletype was a big improvement over
those...  I also briefly used the funny cards with round holes, as well.
So forgive me if i can only send with a straight key on a good day...]

What's confusing is that two sets of impedances are being reported, one for
each loop and the imaginary (reactance) part of each is about 50 ohms (with
a different sign for each) at 137.5 MHz, while real part is comparatively
small.  Huh?

I attempted to connect the two loops together with a short segment of 
transmission line:

       http://www.qsl.net/kd6pag/nec/apt1f.html

That gives me an impedance that seems more reasonable, but the model seems
to jump all over the map when the frequency is varied, unlike other models
i've worked with [see anything else on "http://www.qsl.net/kd6pag/nec/";]. 
The result shown above is also reasonably close to another attempt i made 
to connect the two loops as illustrated on the WWW page.

Please understand that i'm not questioning the real world results which are
being gotten at all.  But i'm having trouble modeling it and/or trying to 
understand the model given on the WWW.

			-- KD6PAG  (Networking Old-Timer, "RF newbie")

Reference:
  NEC-2 User's Guide from <http://www.traveller.com/~richesop/nec/>, 
      CCC's Word/Mac WDBN version 0.92, as presented in .PDF format.
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